This invention relates to the application of protective coatings to the interior seams of cans and, more particularly, to the application of protective coatings to the interior of the welded overlapped seam or the butt weld seam of a three-piece metal can.
Metal cans are made by either one of two processes. One process, the two-piece can process, involves forming a drawn cup from a cylindrical slug of metal and then deep drawing the cup to a can configuration. The other process, the three-piece process, involves forming a cylindrical can body from a sheet of metal and then attaching two lids or ends to the opposite ends of the body. The present invention is concerned only with the application of protective coatings to three-piece cans.
In the manufacture of three-piece cans, the cylindrical bodies of the cans are formed by wrapping a sheet or blank of metal around a so-called stubhorn. The ends of the sheet are either butted or overlapped and secured together by a welded seam, a soldered seam, or a cemented seam. The interior of this seam is then coated with a protective coating, the function of which is to protect the contents of the can against metal contamination. In the application of this coating, continuity is extremely critical because any pinholes, cracks or imperfections in the integrity of the coating will generally render the can unsuitable for most applications.
After application, the coating is cured by heating. This curing process is applied only to the area of the can seam. Therefore, any coating material applied to the interior of the can which is not upon the seam area will not be cured.
The present invention is primarily concerned with applying this continuous coating onto welded can seams. Welded can seams tend to be more brittle and more irregular than soldered can seams, and, therefore, require a generally thicker coating, usually about 6 milligrams of dried resin material per square inch.
Methods capable of applying such a thick coat of material include powder coating and roller coating methods. These methods are extremely expensive and the apparatus is difficult to maintain.
A third method is airless spray. Although an air spray is frequently used to coat a can seam with a thin layer of material, it is impractical to apply a thick coat of material using air spray. The present invention is directed to a method and apparatus for airlessly spraying a thick stripe of a protective coating onto the welded seam of a can body.
Hogstrom U.S. Pat. No. 3,921,570 discloses an apparatus for airlessly spraying a thin coating of material onto a can seam. This apparatus is designed to airlessly spray an atomized mist of material onto a can seam. The apparatus was designed to spray a low solids material which is easily atomized.
To apply a thick coat, a higher solids coating material is needed. Generally, the material is 20 to 40 or more percent solids. Such coating material is not easily atomized. If the apparatus as described in Hogstrom were used with a high solids coating, the coating material would not completely atomize and as a consequence, it would tend to bounce off the seam of the can onto other portions of the can. Consequently, this material, commonly referred to as rebound, would not be cured. The rebound would then itself constitute a contaminant to the contents of the can.
It is an object of the present invention to prevent this rebound. Specifically, the object of the present invention is to airlessly spray a thick coating of a high solids material onto a welded seam of a can body without significant rebound. This is accomplished, according to the practice of this invention, by airlessly spraying the coating material through a nozzle orifice onto a can seam and positioning the nozzle close enough to the can seam that the coating material does not break up or atomize before it hits the can seam. We have found that this invention is in part predicated upon the discovery that by positioning the nozzle close enough to the can seam, a high solids coating material can be airlessly sprayed onto a can seam without significant rebound.
This enables one to apply a thick stripe of coating material onto the seam of a can using airless spray apparatus, which in turn allows one to apply a stripe of controlled width and thickness. Such a system is more reliable than other striping methods presently used to apply a thick stripe of material. Furthermore, the present method of applying a stripe is significantly less expensive than these alternate methods.